1. FIELD OF THE INVENTION
The present invention relates to focus detecting optical system employed, e.g. for an automatic focusing photographic camera, and more particularly to a focus detecting optical system comprising a condenser lens in the vicinity or rearwardly of a predetermined focal plane of a photographic lens, a pair of image forming lenses disposed symmetrically with each other relative to a main optical path and rearwardly of the condenser lens and a light blocking member disposed forwardly of the image forming lenses and defining openings for regulating incoming luminous flux such that a focal point of the photographic lens is detected by comparing images of the predetermined focal plane formed on a light receiving element array by the pair of image forming lenses.
It is to be noted here that the predetermined focal plane denotes a plane positioned from the photographic lens at a distance equal to that between the photographic lens and a film face, i.e. the plane on which an image is always formed with the photographic lens being at its focused condition.
2. DESCRIPTION OF THE PRIOR ART
Various kinds of focus detecting optical systems having the above-described construction have been proposed and have also been embodied thus far. However, the conventional constructions have a problem in that it is impossible to expand a detection sensitivity range in an angle of view because of a characteristic limit of an employed photoraphic lens.
In order to illustrate more fully this problem, a conventional construction will be described firstly with reference to FIG. 5. A primary image I formed in the vicinity of a predetermined focal plane Fo of a photographic lens (not shown) is re-formed by a pair of image forming lenses 17a, 17b through an infrared ray cut filter 12, a condenser lens 13 and a pair of openings 16a, 16b defined in a light blocking member 16, thereby forming two secondary images Ia, Ib on a light receiving element array 20. And, a focal point of the photographic lens is detected by detecting a change in positions of these two secandary images Ia, Ib.
If the detection sensitivity range is doubled for example, the primary image I becomes I' and the secondary images Ia and Ib become Ia' and Ib' respectively, whereby the secondary images Ia' and Ib' partially interfere with each other and the focus detecting operation becomes impossible. One way to avoid this mutual inteference between the secondary images is to increase decentering value Xt of the image forming lenses 17a, 17b from a main optical path `1`. However, if the decentering value Xt is set to be much larger than a decentering value Xm of the openings 16a, 16b, horizontal aberrations such as coma aberration and distortion increase thereby to deteriorate the quality of the secondary images Ia', Ib'. In order to avoid this deterioration of the quality of the secondary images, it is possible to increase both the decentering value Xt of the image forming lenses 17a, 17b and the decentering value Xm of the openings 16a, 16b. However, if the decentering value Xm of the openings 16a, 16b is increased, an inclination .theta. of the luminous flux used for the focus detection relative to the main optical axis `1` is also increased. And, this increase in the inclination .theta. causes vignetting in effective luminous flux in case of a photographic lens having a large F number whereby the focus detection is impossible.
As described above, with the conventional constructions, if the detection sensitivity range is increased, only a photographic lens having a small F number may be used. On the other hand, in order to enable the focus detection with the photographic lens having a large F number, the detection sensitivity range need be decreased. With a small detection sensitivity range, it is impossible to follow a fast moving object since the object easily exceeds the detection sensitivity range or the focus detecting operation is impossible because a sufficientlycontrasted portion is not held within the focus detection sensitivity range with a desired composition.